Compact type electrical discharge device



INVENTORS- David J, Hayley y fPez/erefi faerfsozz June 22, 1954 D. L.HILDER ETAL COMPACT TYPE ELECTRICAL DISCHARGE DEVICE Filed Jan. 11, 1951Patented June 22,1954

COMPACT TYPE ELECTRICAL DISCHARGE DEVICE David L. Hilder, Allentown,Pa., and Revere H.

Robertson, Westfield, N. J., assignors to Hanovia Chemical andManufacturing Company, Newark, N. J., a corporation of New JerseyApplication January 11,1951, Serial No. 205,549

Claims.

The present invention deals with a compact type electrical dischargedevice and more particularly with an electrode therefor.

The compact type electrical discharge lamp is distinguished from thetubular and capillary type discharge lamps in that the lamp envelope isof a substantially spherical configuration and the electrodes containedtherein are spaced to provide a discharge path of from a few millimetersto about two centimeters in length. The electrodes in lamps of this typeare subjected to very high current densities and, especially during thestarting of the discharge when the electrodes are cold, to a highevaporation rate of the metal forming the electrodes, which blaclrensthe walls of the lamp envelopes, e. g. light transmissive envelopes offused silica, fused quartz, or other glasses of high melting point, andthereby decreases the light output of the lamps. The rate at which theelectrodes evaporate is the determining factor of the useful life of thelamp.

Tungsten is generally used as the electrode material. The electronemissivity of tungsten increases enormously with temperature, being ofthe order of one hundred billion times greater at near the melting pointthan at about 25 Centigrade, and the temperatures developed in thedischarge chamber of compact type lamps employing such electrodes areexceedingly high, e. g. between about 5,000 and 11,000 Kelvin. However,tungsten melts at 3,643 Kelvin and the electrodes will melt unlessemployed in very specificforms to dissipate by radiation and conductionthe heat received from the discharge.

Usually, the electrodes used in compact type lamps are massive instructure-in comparison with the electrodes of other type lamps havingenvelopes of comparable size-in order to provide ample surface forradiation because radiation is a surface phenomenon. Massive electrodeshave certain undesirable features in that the initial emissivity is lowand sputtering of the tungsten metal is likely to occur, the extent ofsputtering being dependent in part on the rate at which the metal warms.For the same current, a massive electrode warms more slowly than asubstantially smaller-sized electrode and, therefore, sputters to agreater extent and more rapidly blackens the envelope than the smallerelectrode during the heating up period.

Electrodes of substantially less mass than the aforesaid massiveelectrode will heat faster and sputter less but will evaporate morerapidly after they have been heated and blacken the envelope morerapidly after the heating up period. Moreover, such smaller-sizedelectrodes, due to the rate of evaporation of the tungsten after theheating up period, causes the lamp employing such electrodes to have ashorter useful life than lamps employing the more massive electrodes.

Therefore, specific forms of electrodes for compact type lamps are acompromise between rate of initial sputtering action and subsequentevaporation.

It is an object of the present invention to provide an improvedelectrode for compact type lamps. It is another object of the presentinvention to provide an electrode which combines the advantages ofmassive and substantially smal1ersized electrodes. It is a furtherobject of the present invention to provide a compact type electricaldischarge lamp which employs the improved electrodes for a longer usefullife of such lamps. It is a still further object of the presentinvention to provide a compact type lamp and electrodes therefor havinga combination of embodiments which cooperate to provide a superiorcompact type lamp. Other objects and advantages of the present inventionwill become apparent from the description hereinafter following anddrawings forming part hereof, in which:

Figure 1 illustrate partly in elevation and partly in section a majorportion of a compact type lamp according to the present invention, and

Figure 2 illustrates an enlarged view of the salient features accordingto the present invention.

The present invention is concerned with high pressure and super highpressure rare gas or rare gas and metal vapor compact type dischargelamps having light transmissive substantially spherical envelopes offused quartz and the like glasses, and an improved electrode structurewhich avoids in a large measure the inadequacies and disadvantages ofelectrodes heretofore em ployed in compact type lamps.

Referring to Figure l, the compact type lamp of this invention comprisesa substantially spherical vitreous e. g. fused quartz, envelope idefining a discharge chamber and having tubular fused quartzextensions,e. g. tubes 2 and 3 projecting outwardly therefrom preferably oppositelyof each other, which support a pair of spaced electrodes 4 and 5hereinafter referred to as main electrodes. The main electrodes arespaced from each other in the discharge chamber to provide a dischargepath of from a few millimeters, e. g. 5 millimeters, to about twocentimeters in length. The discharge chamber contains an ionizableatmosphere of a rare gas at at least one atmosphere pressure at about 25C., or a rare gas and a vaporizable metal, e. g. mercury, in an amountsufiicient when vaporized to provide a gas-vapor filling of at least oneatmosphere total pressure at about 25 C.

The improved electrode structure of the present invention comprises theaforesaid pair of spaced main electrodes 4 and 5 upon each of which ismounted an arrangement of auxiliary-electrodes. Each main electrodeconsists of a rod of substantial mass, e. g. a rod having adiameter ofapproximately 0.004 inch per ampere of normal current rating for theelectrode, said current being in the order of at least 5 amperes for acompact type lamp. The length ofleach main electrode or rod is anessential feature in that it is at least three times the length of theinside radius of the spherical envelope and a major portion of the rodis encased or sealed and supported by a vitreous or glass tube orextension, e. g. tube 2, and a minor portion of the rod extends into thedischarge chamber, said major portion acting to conduct heat from saidminor portion and said glass tube assisting in the conduction of heataway from the discharge and electrode and thereby cooling the electrodeby conduction and convection. It is apparent that the tubes 2 and 3,since they seal-in major portions of the electrodes 4 and 5, arethemselves of substantial length thereby allowing sufficient conductionand convectionto prevent at least the ends of the rods 4 and 5 outsidethe discharge chamber from incandescing. If the length of the electrode4 sealed into the tube 2 is too short relative to that portion extendinginto the discharge chamber, the arcing ends of the electrode willoverheat, possibly melt, and in such case will evaporate rapidly andblacken the walls of the envelope. Accordingly, if the length of themajor portion of the electrode outside the discharge chamber is at leastthe length of the electrode, preferably from about to the length,suflicient conduction and convection are maintained to prevent the abovementioned detrimental conditions. However, in regard to the lower limitsof lamp temperature and the diameter of the electrode, in some instancesit is sufficient that a major portion of the electrode is outside thedischarge chamber.

The main electrodes .are preferably composed of commercial tungstenwithout the addition of activators since the latter are not satisfactorybecause they evaporate too readily at the operating temperature of theelectrode, whichat its arcing end is at about 3400" Kelvin. It isapparent that the major portion of the main electrode is not sealedvacuum tightly. Therefore, a vacuum tight seal is connected to the endof the main electrode which is outside the discharge chamber. Thisvacuum tight seal is sealed by said tube l as is the main electrode andis electrically conductive and of sufficient capacity to carry thenormal current rating forthe electrode.

For example, the vacuum tight .seal may be in the form of asubstantially thin strip .or sleevetype structure 6 as illustrated andcomposed of molybdenum and the like materials known to the art toconstitute suitable materials forzsuch seals.

The minor portion of both main electrodes. which extend into thedischargechamber, have mounted thereon an arrangement .of firstauxiliary electrodes 1 and 8 which assist the start of the discharge andsecond auxiliary electrodes 9 and I0 which warm rapidly to take up thedischarge from electrodes 1 and 8 and sustains the discharge until theends H and I2 are heated sufficiently to take up the discharge toestablish a fixed arc. The whole starting procedure including theestablishment of the fixed arc occurs in a fraction of a second when alamp employing such an electrode structure is connected to a suitablepower supply.

In order to carry out the objectives hereinbefore set forth and toenable the electrodes to operate as above described, it is essentialthat the main electrode and the auxiliary electrodes supported thereonform an electrode structure which comprises a combination of the saidelectrodes in a substantially critical relationship to each other. Sincethe minor portions of main electrodes 4 and 5, particularly the ends Hand 12, are relatively massive they can operate and sustain a dischargeare without evaporating sufficiently to render the lamp susceptible to ashort useful life. However, the second auxiliary electrodes, in order tooperate as described must be of less mass than the minor portions of themain electrodes in order to heat up rapidly enough to take up thedischarge from the first auxiliary electrodes almost instantly, and mustnot be positioned too near to the fixed are sustained by the mainelectrodes to prevent the evaporation thereof. Therefore the secondauxiliary electrodes 5: and I6 are supported on the minor portions ofthe main electrodes but spaced from the arcing ends I i and i2. Thesesecond auxiliary electrodes are preferably spaced from the ends I i andi2 for a distance relative to the difference in mass between the twosolid electrodes. For example, as a preferred embodiment the secondauxiliary electrodes may comprise tungsten coils of about 5 to 7 turnseach and the diameter of the wire forming said coils being aboutfiveeights the diameter of the main electrodes, said coils being spacedfrom the ends Ii and 2 at a distance approximately twice the diameter ofthe main electrode not only to prevent the evaporation thereof, but toalso conduct and radiate energy conducted to them from the ends i i andI2 and to thereby extend the useful life of the discharge device.

The first auxiliary electrodes 1 and 8 are essentially of less mass thanthe second auxiliary electrodes and, although preferably in the form ofa round wire coil of tantalum or molybdenum, may be of any form as abovespecified with regards to the second auxiliary electrode. fantalum ormolybdenum is preferably used for the first auxiliary electrode becausethese metals have better electron emission qualities when cold than doestungsten they possess a sufficiently high melting point to exist in anunmelted form when positioned or supported on the minor portion of themain electrodes near or in contact with the inner wall of the quartzenvelope 1 and between the inner envelope wall and at least some of theturns of the second auxiliary electrode. Other materials are betteremitters than tantalum and molybdenum, e. g. thorium, akaline earths,lead, etc, but they all evaporate rapidly at the temperaturesencountered on the main electrodes. Preferably, the first auxiliaryelectrodes consist of a few turns each of round wire having .a diameterless than that of the second auxiliary electrodes, e. g. about oneeighththe diameter of the main electrodes,.and are in electrical contact withthe main electrodes as are the second auxiliary electrodes and theauxiliary electrodes may or may not be in contact with each other.Moreover, since the first auxiliary electrodes are positioned fartherfrom the ends of the main electrodes than are at least some of thewindings of the second auxiliary electrodes, they are shielded from thetemperatures of the are between the said ends by the said secondauxiliary electrodes which at least to some extent contribute to preventthe said first auxiliary electrodes from reaching excessivetemperatures.

In accordance with the present invention hereinbefore set forth, acompact type discharge device employing the above described electrodestructure has a greatly reduced electrode evaporation rate, consequentlya reduction of blackening of the lamp envelope, providing for a longeruseful life of compact type discharge devices.

What we claim is:

1. A compact type electrical discharge device comprising a substantiallyspherical vitreous envelope defining a discharge chamber and having apair of tubular vitreous extensions projecting outwardly therefrom, saiddischarge chamber containing an ionizable atmosphere, a pair ofelectrically conductive rods, each of said rods having a length equal toat least three times the length of the inside radius of said envelope, amajor portion of each of said rods being contained and sealed by each ofsaid extensions, a continuous minor portion of each of said rodsextending into said discharge chamber, said minor portions being spacedfrom each other for supporting a discharge arc, each of said minorportions comprising a main electrode, first and second discontinuousauxiliary electrodes supported on said main electrode in electricalcontact therewith, said second auxiliary electrode being of less massthan said main electrode and spaced from the are supporting end of saidmain electrode, said first auxiliary electrode being of less mass thansaid second auxiliary electrode and positioned between the inner wall ofsaid envelope and at least a part of said second auxiliary electrode.

2. A compact type electrical discharge device according to claim 1,wherein said vitreous extensions project outwardly of said envelope anddiametrically of each other, and said minor portions of said rods beingspaced less than two centimeters of each other.

3. A compact type electrical discharge device according to claim 1,wherein said ionizable atmosphere comprises a rare gas and sufficientmercury when vaporized to provide a total pressure of at least oneatmosphere at about 25 C.

being tungsten bodies, and each of said first auxiliary electrodes beingtantalum bodies.

5. A compact type electrical discharge device comprising a substantiallyspherical vitreous envelope defining a discharge chamber and having apair of tubular vitreous extensions projecting outwardly therefrom, saiddischarge chamber containing an ionizable atmosphere, a pair of tungstenrods, each of said rods having a length equal to at least three timesthe length of the inside radius of said envelope, a major portion ofeach of said rods being contained and sealed by each of said extensions,a continuous minor portion of each of said rods extending into saiddischarge chamber, said minor portions being spaced from each other forsupporting a discharge arc, each of said minor portions comprising amain electrode, first and second discontinuous auxiliary electrodes mainelectrode in electrical supported on said contact therewith,

said second auxiliary electrode being a tungsten wire coil co-axial withsaid main electrode and the diameter of the wire of said coil being lessthan the diameter of said main electrode, said second auxiliaryelectrode being spaced from the are supporting end of said mainelectrode, said first auxiliary electrode being a wire formed 01 atleast one of the metals tungsten and tantalum, the wire of said firstauxiliary electrode being of less diameter than that of said secondauxiliary electrode, said first auxiliary electrode being positioned onand co-axial with said main electrode between the inner wall of saidenvelope and at least part of the windings of said second electrode.

References cited in the file of this patent UNITED STATES PATENTS NumberNumber

